Body mass index is not the best measure of health, according to the first real-world test of a new way to assess obesity, published this week in the journal Nature. The study found that the risk of death was better predicted by excess body fat than by BMI alone, adding weight to a debate that has run for years about what the familiar number can and cannot tell a doctor.
Francesco Rubino and colleagues analyzed health data from 844,108 adults across four groups in China, Britain, the United States and Spain. They tested a framework proposed by a 2025 Lancet Commission, which said BMI alone should never be used to diagnose obesity. The researchers sorted people by body fat and by signs of organ damage, then tracked who died. Excess body fat predicted death more accurately than body mass index at every level.
The result lands in the middle of a shift in how medicine thinks about weight. The Lancet Commission proposed two new categories: pre-clinical obesity, where excess fat raises the risk of disease, and clinical obesity, where excess fat has already damaged organs or limited daily life. More than 75 international medical organizations signed on to the commission’s report. The idea is to diagnose the condition, not the number.
Why body mass index misses the mark
Body mass index was never built for this job. Belgian mathematician Adolphe Quetelet devised the formula in the 1830s as a way to describe the average man in population statistics, not to judge the health of one person. Physiologist Ancel Keys repurposed it in 1972 as a quick screen for obesity in large groups, and he said directly that it should not be used to evaluate individuals. Clinics, insurers and health apps kept it anyway because it needs only height and weight.
The formula divides weight by height squared, and that is all it does. It cannot tell muscle from fat, so athletes and people with heavy muscle often land in the overweight range. It cannot show where fat sits on the body, and that matters: fat wrapped around organs in the belly is tied to heart disease, diabetes and dementia, while fat under the skin on the hips and thighs carries far less risk. Body mass index lumps the two together.
The cutoffs are not the same for everyone, either. A World Health Organization review found that people of Asian descent can carry a raised risk of type 2 diabetes and heart disease at a BMI below 25, the usual line for overweight. In 2023, the American Medical Association called the measure problematic, citing its historical harm, its use for racist exclusion, and the fact that it rests on data drawn mostly from earlier generations of non-Hispanic white people.
What body mass index cannot see
Large studies have shown how often the number misleads. A meta-analysis of 25 studies covering 31,968 people found that BMI caught only about half of those who truly had high body fat. In a large United States survey of 40,420 adults, researchers measured blood pressure, blood lipids, blood sugar, insulin resistance and inflammation directly. They found that nearly half of the people classed as overweight by body mass index showed no signs of organ trouble, while about 29 percent of those with a normal body mass index were metabolically unhealthy.
The gap runs both ways. A person with a low number can carry dangerous fat around the organs and still look fine on paper. A person with a high number can be muscular, active and healthy by every blood test. Doctors who rely on the number alone can miss the first group and treat the second group for a problem it does not have.
Better ways to measure health risk
Researchers point to measures that capture where fat sits. Waist-to-hip ratio divides waist size by hip size and predicts heart attacks better than body mass index. Waist circumference on its own is a stronger guide to health risk than body mass index. The body roundness index uses height, waist and weight to model body shape, and studies show it estimates visceral fat more accurately than any single measurement. The weight-adjusted waist index divides waist size by the square root of body weight to capture central fat.
None of this makes body mass index useless. It is cheap, fast and reasonable for describing groups, and it still has a place as a first screen. What the new evidence shows is that it should not be the last word. A waist measurement, a body composition scan and blood work tell a doctor more about one person’s health than a single number from a scale.